Reclaiming mixtures of vulcanized natural rubber and synthetic rubber scraps



Patented Sept. 30, 1952 UNITED STATES earENTj O ER.

RECLAIMING MIXTURES OF VULCANIZED NATURAL RUBBER AND SYNTHETIC RUB- BER SCRAPS Walter G. Kirby, Naugatuck, and Leo E. ,Steinle, 7 Beacon Falls, Conn., assignor's to United States Rubber Company, New York, N. Y., a corpora v tion of New Jersey No Drawing. Application August23, 1950,

Serial No. 181,104

This invention relates to a process for reclaiming mixtures of Vulcanized natural rubber and vulcanized butadiene-styrene copolymer synthetic rubber scraps.

Vulcanized butadiene-styrene copolymer synthetic rubber scrap is much more difficult to reclaim or soften to a millable condition than vulcanized natural rubber scrap. As a result, when a mixture of vulcanized butadienestyrene co' polymer synthetic rubber and vulcanized natural rubber scraps is subjected to heat and plasticiz ing agents to such an extentthat the naturalrubber portion becomes sufficiently soft 'and mill-. able for a satisfactory reclaim, the synthetic rubber portion remains comparatively hard. When attempts are made to mill or refine such a reclaim mixture to a smooth sheet, the harder re claimed synthetic rubber particles produce a roughness in the blend which results in a nonuniform and inferior grade of reclaim. Prior-to the use of synthetic rubbers practically all vulcanized scraps were natural rubber. A great majority of such natural rubber scrap was reclaimed by the alkali digester process. The alkali type natural rubber reclaim had a higher'valuethan other types of reclaims because it had a f ast-- er rate of cure and a general similarity to crude rubber in properties such as tackiness, elasticity; tensile and milling. When vulcanized synthetic rubber scrap became available, the alkali process proved unsuitable for its reclaiming and as a result gradually went out of use. The reclaiming process now generally used forvulcanized syn-' thetic scraps and for mixtures "of the same with natural rubber scraps, is carried out in an acid or neutral medium. Reclaims produced by these 8 Claims. (01. 26452.3?

methods lack many of the valuable properties of alkali type natural rubber reclaim in that they are slower in rate of cure and have less tacki-' ness, elasticity and tensile.

An object of the present invention is to provide an improvement in the process of reclaiming mixtures of vulcanized natural rubber and vulcanized butadiene-styrene copolymer synthetic rubber scraps.

Another object of the present invention is to provide a method of reclaiming mixtures of vulcanized natural rubber and vulcanized b'utadi ene-styrene copolymer synthetic rubber scraps in contact with each other and in the same mass to produce a reclaim product that in quality and general properties resembles an all natural rubber alkali type reclaim. Q

A-further object of the present invention is to provide an improved method of plasticizing r mixtures of vulcanizefd natural rubber and vul canized butadiene-styrene copolymer synthetic rubber scraps so that each" type will be plasticized or softened to a sufficiently uniform, degree of viscosity and the reclaimed rnixture will better mill or refine to produce a satisfactory uniform homogeneous finished reclaim product.

. In carryingout the present mvention a'mixture of vulcanized natural rubber scrap and Vulcanized butadiene-styrene copolyrnersynthetic rubber scrap is first subjected to heat" and conventional plasticizing agents in'a rion-alkaline medium until the synthetic rubber portionis sufiiciently softand millable'for a satisfactory reclaim and the" natural rubber portion is in too softa condition for a satisfactory reclaim and any fibers present have been 'disintegratedf There is then added to the mixed reclaim an agent which at elevated reclaiming temperatures will harden the natural rubber portion at a faster rate than the synthetic rubber portion, so, that in arelatively, short period of time the extremely soft natural rubber. portion will be hardened e} sufiiciently to make a satisfactory reclaim' and the synthetic rubber portion will be hardened to a lesser extent and will remain suff ciently soft fora satisfactory reclaim. We have found that sodium hydroxide and certain other alkaline materials are such rubber stiffening agents as will impart a difierential increase'in hardness tonatural rubber and butadiene-styrene copolymer synthetic rubber, making the process of the present invention possible. 'Alkalinestiffening agents other than sodium hydroxide that may be used in the present invention are potassium hydroxide, calcium hydroxide; ammonium hydroxide,- sodium carbonate, potassium carbonate, tris'odium phosphate, tripotassium'-phosphate;fso dium sulphite; potassium sulphite.

hardening or stiffening agent used is not critical. Generally, the amount of stiffeningagent added will be between 2*and10 percent by'we ight of the original mixed scraps. The treatment at elevated temperature of the mixed softened "scraps with the sodium hyd'roxideor other alkaline'stiffening" agent is only for the short timenec'essary" to f harden the rubbers, after which the alkalinemaf terial may or may not be washed'out as in the usual manner with alkali reclaimsgiving a product. which, will mill and smooth out readily into a relatively uniform, high quality, satisfactory sheet. The time of treatment of' the; mixed softened scraps with the sodium hydroxide 'or other stiffening agent will be a fraction; generally 7 Mixtures of. such alkaline -materials maybe used. The amount of such 3 less than one-half the time for softening the scraps and destroying the fibres. The butadienestyrene copolymer synthetic rubber in the scrap to be reclaimed is generally from GR-S synthetic rubber which contains a major proportion of butadiene-l,3 and a minor proportion of styrene.

, ride, and dried and milled. The temperature in The weight ratio of such synthetic rubber to natural rubber in the scrap may be in the range from 1:9 to 9:1.

The reclaiming process of the present invention may be carried out in conventional reclaiming apparatus, for example, asused in the digester or mechanical reclaiming processes. In

conventional reclaiming processes, ground vulcanized rubber scrap is subjected to heat and in the case of the mechanical process to heat and mechanical working. It is customary to use plasticizing agents such as softening oils and, if desired, chemical plasticizers and fiber destroying agents to accelerate the disintegration of cellulose contained in thescrap. The rubber is thus softened down to a workable condition and the I fiber present is destroyed. The effectiveness of recl imi era in r s e ins sc ap vulcanized rubber may be quantitatively shown by means of an'instrument widely used for this purpose in the reclaim rubber industry, namely, the M o S e ring c) p a tom ter. has been described by Mooney in Industrial and Engineering Chemistry An. Ed., 6, 147 (193.4); By means of this device the viscosity of 'a'plastic material in shear may be readily-and quantitatively measured. Experience has shown This device that reclaims with a Mooney viscosity'of 50 to 180 I measured ,at; 180 F. can be processed on standard rubber working machinery. Reclaims with aMooney'vis'cosity of 80 to 120 measured at 180 Ffare {preferred since these are the most practicaltojprocess. Reclaims'with a higher Mooney viscosity than 18.0 measured. at 180 F. are too hard, andreclaims with a lower Mooney viscosity than .50. measured at 1 809 F.'are too soitior efflcient processing. Thus when niixturesv of natural rubber scraps and synthetic rubber scraps are reclaimed by conventional processes, if the reolaimingis-carried to the extent to give a satisiactory Mooney viscosity to the natural rubber portion, the syntheticrubber portion will be too hard, and if the reclaiming is carried tothe extent to give a satisfactory Mooney to the syntheticrubber portion, .the natural rubber portion wi be t d Q h' fini hed m l d re lai n the ig ime wi be t o ou h beca s o t e a de re lai ed s nt eti r b a tic e a in thelatter casethey Willlbe too, soft and .,soupy from-the,excessivelysoft natural rubber pbl'tion.

.Ilhe present inventionnarrows the rangebetween the container .or fdig'ester? equipped with an agita tor'until the cellulose fiber in thefscrap is destroyed and the rubber becomes .plastioiz'ed. The charge may then .be.washed free from added chem al isu hia di m hyd o ide o zi c 0.1 1

the digester is from 300 F. to 430 F., and the time of treatment is several hours. In the process of the present invention, the initial treatment in the digester is in a neutral or acid (nonalkaline) solution, at the usual digester reclaiming temperatures from 300 F. to 430 F.

for several hours to destroy the fiber and to excessively soften the natural rubber portion to an unsa'tisfactory'soft condition, i. e., to a Mooney viscosity measured at 180 F. between about 20 and about 40. At the same time, the synthetic rubber portion which softens with more diificulty will be softened to a satisfactory Mooney viscosity measured at; 180 F. between about 50 and about 140. The alkaline stiffening agent, such 1 as sodium hydroxide, is then added to the digester charge with agitation continued for 10 minutes to an hour, which will harden both the natural rubber portion and the synthetic rubber portion, but the former much more than the latter, so that the Mooney viscosities measured at 1e09 F. of the blend of the natural rubber scrap particles and the synthetic rubberfscrap particles 7 will be in the normal range of 50 to 180 for satisfactoryreclaim. This short after-treatment with, sodium hydroxide is entirely different from the extended digestion of scrap in the presence of sodium hyd TQ ide in the alkali reclaim process,

or the extended digestion-of fiber-containing re-' claim in aqueous sodium hydroxide to destroy fibers not destroyed in a prior reclaiming operation. In these cases, the extended digestion in the caustic alkali solution containing the conventional plasticizers overcomes any inherent stifiening action of the sodiumv hydroxide. I

In the conventional mechanical process, the dry vulcanized fiberecontaining or fiber-free rubber scraps and plasticizing agents are subject to increasing temperature in an internal mixer having a jacket through which hot water flows,

such as a Banbury mixer, under mechanical pressure exerted by a rain upon the scrap whichis worked upon by rotors into the mixer chamber.

The temperature may rise to 300 F. to 550 The time of treatment may range from. 5 to minutes ter wh ch he ar e ay be c ol y flowing coo in Wa er rough t e jacketed.

and rot rs f the mi er and by add: ns wa er to th char e. In h ocess o he pre t -i- .ventio.n th mix d u i ed natral rubb r an synthe rub e c p adixed w th co.nv n b alv as ibiz ns a ent -and n a n n-a ha n c dition are heated n t e Ban-bury at th usualtbmper tures ween 3.0 and 5. 05 with' qrkin until the fi er pre nt is;giisiri ca atcs1v nd he atura u be poron is so sued t a Moone viscosity m asured M1 F- tween. a'b utZ and ab ut bi may take 5 to 60 minutes. At the same time. the nth ti rubber port on ich. softens with more iflijculty will have a satisfactory n yv vissi y .mcasn fedat 11180? F. "between about 5.0 and.

bout 1.40- dium hydroxide or ther alkaline stiffening agent is then incorporated in the soften d char e i the .Banbury as by adding itv in iution inrthe water and. m xing it in for :afeW inutes- .The mixing ime may be from 2 $0110 minutes, and. is less than the plasticizing'time eiqr he llsaliad ition, reclai th n e. hea e to a satisfac or un o m she t- 7 Inpractice it is not necessary .t follow each run by a determination of the MOOney viscosities of the natural and synthetic portions of the re-" are-". ar er were to d t m na e Mooneyviscosity of the final product before milling and refining. Reclaiming rubber is "practically a continuous operation and an experiencedv operator knowing the Mooney viscosity of the mixed reclaim prepared according to theapresent invention in a particular run can compensate subsequent runs so that the finished stock has a Mooney viscosity which is within the range-of 50 to 180 measured at 180 F., and generally within the preferred range of 80 to 120 measured at The following examples illustrate theinvention, all parts and percentages referred to herein being by weight: 1

Example 1 Ten thousand pounds of ground scrapcontaining about 25% of fibers and the rubber portion. comprising about 80% of vulcanized natural rubber and about. 20% of vulcanized GRi-S synthetic rubber were placed in a digester with 1000 pounds of pine oil fraction and 800 pounds of rosin oil (softening oils), 100 pounds of technical discresyl sulfide (chemical plasticizer), 100 pounds of calcium chloride (cellulose defiberizing agent), and 20,000 pounds of waters The .digester was sealed and heated under pressure at about 388 F. for about 5 hours. The cellulosefiber was destroyed, and the Mooney viscosities measured at 130 F. of the natural rubber portion was about 20, and of the synthetic rubber portion was about '70. An alkaline stiffening agent, viz. 6l0pounds of sodium hydroxide was then introduced into the digester charge and the heating and mixing continued for 30 minutes at 388? F. After this hardening treatment the Mooney viscosity measured at 180 F. of the natural rubber portion was about 70 and that of the synthetic rubber portion was about 100, giving a satisfactory Mooney viscosityrangeofbO I to 180 for the natural rubber and synthetic rubber portions, and for the blend. The reclaim from the digester was washed, dried and milled on tight mill rolls to give a satisfactory smooth reclaim. The .Mooney viscosity measurements above were made on actual separated natural rubber and synthetic rubber scrap particles.

Example II One hundred thirty-one pounds of ground scrap containing about 25%, of fibers and the rubber portion comprising about 80% of vulcanized natural rubber and about 20% of vulcanized GR-S synthetic rubber scrap, 9 pounds of coal oil fraction, 9 pounds of rosin oil, 6 pounds of carbon black, and 3.5 pounds of rosin oil, were loaded into a Banbury mixer used for mechanica reclaiming of rubber which was heated by pressure and friction from mechanical action. The charge was masticated in the Banbury under about 200 pounds per sq. in. ram pressure for 16 minutes,,the' temperature of the charge reaching about 500 F., whereupon the fiber was destroyed and the Mooney viscosities measured at 180 F. of the natural rubber portion was about 20, and of the synthetic rubber portion was about 80. Three and one-half pounds of 50% aqueous sodium hydoxide solution at 80 F. was intro duced into the Banbury charge along with about one gallon of cooling water and the mixing continned for 3 minutes, after which the batch was discharged as a satisfactory mixed reclaim.

The plasticizing agents used in the present invention are those conventionally used in reclaiming operations and the amount is not critical.

Generally, lto 25 per cent of the weight of the scrap tobe reclaimed may be conventional softeningoils. If a conventional chemical plasticizer is also used, it will generally be in amount up to 10 per cent of the weight of the scrap. Examples of known chemical plasticizers that may be added to aid in softening the scrap are thiophenols, hydrazines, alkyl amines, aliphatic polyamines, mercaptans, di(hydroxyary1) sulfides, thiophenes, organic acids, etc. Where a fiber disintegrating agent, such as zinc chloride or calcium chloride is added to aid in the destruction of the fiber, the amount used may be up to 3% of the weight of the conventional fiber-containing scraps. In any case, according to the present inventiomthe fiber present inthe scrap is disintegrated in the initial rubber softening treatment in a neutral or acid medium, and the alkaline stiffening agent, such as sodium hydroxide, is added to the reclaim after the fibers have been disintegrated for the purpose of differentially hardening or stiffening the softened natural rubber and synthetic rubber portions.

In view of the many changes and modifications that maybe made without departing from the principles underlying .the invention, reference should be made :to .the appended claims for an understanding of the scope of the protection afforded the invention.

Having thus described our invention what we claim and desire to protect by Letters Patent is:

1. A process for reclaiming mixtures of Vulcanized natural rubber and vulcanized butadienestyrene copolymer synthetic rubber scraps which comprises heating the scraps in a non-alkaline medium for a time sufiicient to soften the natural rubber portion to an excessively soft'condition where the Mooney viscosity measured at 180 F. is between about 20 and about 40, and to soften the-synthetic rubber portion to asatisfactory soft condition where the Mooney viscosity measured at 180.- F. is between about 50 and about 140, and heating said softened mixture with materials, selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide, sodium carbonate, potassium carbonate, trisodium phosphate, tripotassium phosphate, sodium sulphite, and potassium sulphite, whereupon the natural rubber portion will increase in viscosity at a faster rate than the synthetic rubber portion to. give a more uniform reclaim having Mooney viscosity measured at 180 F. in the range between 50 and 180.

2. A process for reclaiming mixtures of vulcanized natural rubber and vulcanized butadienestyrene synthetic rubber scraps which comprises heating the scraps in a non-alkaline medium-for a time sufficient to softenthe natural rubber portion to an excessively soft condition where the Mooney viscosity measured at. 180 F. is between about 20 and about 40, and to soften'the synthetic rubber portion to a satisfactory soft'condition where the Mooney viscosity measured at 180 F. is between about 50 and about 140, and heating said softened mixture with sodium hydroxide whereupon the natural rubber portion will increase in viscosity at a faster rate than the synthetic rubber portion to give a more uniform reclaim having a Mooney viscosity measured at F, in the range between 50 and 180.

3. A process for reclaiming mixtures of fibercOntaining vulcanized natural rubber and vulcanized butadiene-styrene copolymer synthetic rubber scraps which comprises heating the scraps in a non-alkaline medium for a time sufficient '7 to destroy the-fibers in the scrap, and-to soften the natural rubber portion to an excessively soft condition where the Mooney viscosity measured at 180 'F. is between about 20 and about 40,.and to soften the synthetic rubber portion tea-satisfactory soft condition where the Mooney viscosity measured at 180 F, is between about .50 and about1140, and heating said softened mixture with materials selected from the group consisting of sodium hydroxide, potassium hydroxide, :calcium hydroxide, ammonium hydroxide, sodium carbonate, potassium carbonate, trisod-ium phosphatejtripotassium phosphate, sodium sulphite, and potassium sulphitawhereupon the natural rubber portion will increase .in'viscosity'at 'a faster rate than the synthetic .rubber iportionito'givexa more uniform reclaim: having a Mooney viscosity measured at 180 F. in the range betweent50. and 180.

4. ,A "process for reclaiming mixtures of 'fib'ercontaining vulcanized natural rubber. and vulcanized butadiene-styrene copolymer synthetic rubber scraps which comprises heating the scraps in a non-alkaline medium fora time sufficient to .destroy'thefibers in the scrap, andto soften the natural rubber portion to an excessively soft condition where the Mooney 'viscositymeasured at 180? F. is between about 20 and about "40., and

to soften the synthetic rubber portion to a satisfactory soft condition where the Mooney-viscosity measured at 180F. is between ab.out;50 and about 140, and heating said softened "mixture with sodium hydroxide whereupon the natural rubber portion will increase in viscosity at a faster rate than the synthetic rubber portiontc :give a more uniformreclaim having :a Moone'y viscosity measured at 180 F. in the range betweeni50 and 180.

5. A process for reclaiming mixtures of fibercontaining vulcanized, natural rubber and vulcanized butadiene-styrene copolymer synthetic rubber scraps which comprises heating the scraps in a neutral medium for a time. sufficient todestroy the fibers in the scrap, and to soften'the natural rubber portion to an excessively soft condition where the Mooney viscosity measuredat 1 80. F is between about 20 and about 40, and to soften the synthetic rubber portionto .a satisfactory soft condition I where the Mooney viscosity "measured at 180 F. is between about E50"and about 140,. and heating said softened mixture withmaterials selected from the group consisting of sodium Ihydroxide, potassium-hydroxide, calcium hydroxide, ammonium hydroxide, sodium carbonate, .potassium carbonate, trisodium phosphate, 'tripotassium phosphate, sodium sulphite, andzpotassium sulphite, whereupon the natural rubber portion will increase in viscosity at a faster rate than the synthetic rubber portion to give atmorezuniform reclaim having a Mooney viscosity measured :at

180 F. in therangebetweenfiO and 1'80.

6. A process for reclaiming mixtures :of fibercontaining vulcanized natura-l'rubber-'and'vulcanized butadiene-styrene copolymer synthetic'rubber scraps which comprises heating-the scraps in a neutral medium fora time sufficient to destroy the'fibers in the scrap, and to softenthe natural rubber portion to an excessively soft condition where the Mooney viscosity measured-at 180 .F.

is between about 20 s and about 40., andtosoften theisynthetic rubber portion to a satisfactory soft :condition where theiiMooney viscosity measured sit- F. is between about 50 andabout 14-0,:and heating-said softened mixture with sodiumhydroxide whereupon the natural rubber portion will increase in viscosity at a fasterrate than the synthetic rubber portion to give a more uniform reclaim'having a Mooney viscosity measured at 180:F. in'the'range between 50 and 180. a

7. A process for reclaiming mixtures of fibercontaining vulcanized natural rubber and'vulcanized butadiene-styrene copolymer synthetic rubber scraps which comprises heating thescraps Lin an acid medium for a time sufficient to destroy the fibers in the scrap, and to soften the natural rubber portion to an excessively soft condition where-the-Mooney viscosity measured at 180*1. is between about 20 and about 40, and to soften the synthetic rubber portion to a satisfactory soft condition where the Mooney viscosity measured at 180 F, isbetween about 50 and about 14'0,-and

heating said soften-ed mixture with materials sean acid medium for a time sufi'icient to destroy the fibersin the-scrap, and to soften the natural rubber, portion toan excessively soft condition where'th'e Mooney viscosity measured at 180 F. is between about 20 and'about 40, and to soften the synthetic rubber portion to a satisfactory soft condition where the Mooney viscosity measured at 180 is betweenrabout'50 and about 140, and heating said softened mixture with sodium hydroxide whereupon the natural rubber portion I will increase in viscosity at a faster rate than the synthetic rubber portion to give a more uniform reclaim having a Mooney Viscosity measured at 180 F. in the range between 50 and 180.

WALTER "G. KIRBY;

LEO STEINLE.

REFERENCES CIT-ED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date "2,471,496 Randall May 31, 1949 2,522,069 Staten et a Sept. 12, 1950 2,56%802 Carr Sept. 11, I951 FOREIGN BATENTS Number Country Date Great 1 Britain :Dec, 31, .1943 

1. A PROCESS FOR RECLAIMING MIXTURES OF VULCANIZED NATURAL RUBBER AND VULCANIZED BUTADIENESTYRENE COPOLYMER SYNTHETIC RUBBER SCRAPS WHICH COMPRISES HEATING THE SCRAPS IN A NON-ALKALINE MEDIUM FOR A TIME SUFFICIENT TO SOFTEN THE NATURAL RUBBER PORTION TO AN EXCESSIVELY SOFT CONDITION WHERE THE MOONEY VISCOSITY MEASURED AT 180* F. IS BETWEEN ABOUT 20 AND ABOUT 40, AND TO SOFTEN THE SYNTHETIC RUBBER PORTION TO A SATISFACTORY SOFT CONDITION WHERE THE MOONEY VISCOSITY MEASURED AT 180* F. IS BETWEEN ABOUT 50 AND ABOUT 140, AND HEATING SAID SOFTENED MIXTURE WITH MATERIALS SELECTED FROM THE GROUP CONSISTING OF SODIUM HYDROXIDE, POTASSIUM HYDROXIDE, CALCIUM HYDROXIDE, AMMONIUM HYDROXIDE, SODIUM CARBONATE, POTASSIUM CARBONATE, TRISODIUM PHOSPHATE, TRIPOTASSIUM PHOSPHATE, SODIUM SULPHITE, AND POTASSIUM SULPHITE, WHEREUPON THE NATURAL RUBBER PORTION WILL INCREASE IN VISCOSITY AT A FASTER RATE THAN THE SYNTHETIC RUBBER PORTION TO GIVE A MORE UNIFORM RECLAIM HAVING A MOONEY VISCOSITY MEASURED AT 180* F. IN THE RANGE BETWEEN 50 AND
 180. 